1. Field of the Invention
This invention relates to a mold to be used for preparing an optical element comprising a glass such as lens, prism, etc. by press molding of a glass preform, and also to a process for preparing the above mold and a process for preparing an optical element.
2. Related Background Art
The technique for preparing a lens by press molding of a glass preform without requiring polishing steps, which obviates the complicated steps required in preparation of a lens in the prior art and enables preparation of a lens simply and inexpensively, has been recently used for preparation of an optical element comprising a glass including not only a lens but also a prism, etc. and others.
The properties required for such mold material to be used for press molding of an optical element are excellent hardness, heat resistance, releasability, mirror workability, etc. In the prior art, there have been a great number of proposals for this kind of mold material such as metals, ceramics and materials coated with them. To mention some examples, 13Cr martensite steel is disclosed in Japanese Laid-open Patent Publication No. 49-51112, SiC and Si.sub.3 N.sub.4 in Japanese Laid-open Patent Publication No. 52-45613, a material of super-alloy coated with noble metal in Japanese Laid-open Patent Publication No. 60-246230, and also a material coated with a diamond thin film or a diamond-like carbon film in Japanese Laid-open Patent Publication No. 61-183134.
U.S. Pat. No. 4,481,023 discloses tungsten carbide as the material for the mold for molding to be used in a process including the steps of:
(a) preparing a glass preform having a shape approximate to the shape of a desired final product; PA1 (b) preparing a cast mold having an internal shape corresponding accurately to the shape of the desired final product; PA1 (c) exposing said glass preform to a temperature at which its viscosity exists within the range of 10.sup.8 to 10.sup.12 ; PA1 (d) exposing said cast mold to the same temperature as said glass preform or approximate thereto; PA1 (e) arranging said preform in said cast mold while said Glass preform exists within said viscosity range, applying a load on said cast mold over sufficient period of time thereby to make the temperature of said cast mold at least in the vicinity of said mold substantially the same as said preform, and also molding said preform into a shape coincident with the internal shape of said cast mold; PA1 (f) removing the Glass molded product from said cast mold while it exists at a temperature where its viscosity is smaller than 10.sup.13 poise; and PA1 (g) annealing the Glass molded product.
However, 13Cr martensite steel has the drawbacks that it is susceptible to oxidation, and further that Fe is diffused into Glass at high temperature to color Glass. On the other hand, SiC, Si.sub.3 N.sub.4, which are Generally accepted to be oxidized with difficulty, also suffer from oxidation at high temperature to form a film of SiO.sub.2 on the surface, whereby fusion occurs with Glass, and further have the drawback that workability of the mold itself is very poor on account of high hardness. The material coated with a noble metal will cause fusion with difficulty, but since it is very soft, it has the drawback that it is liable to be damaged and deformed. Also, the material coated with a diamond film lacks smoothness of the surface and therefore mirror surface characteristic of an optical element is lacking.
The film called diamond-like carbon film is not limited to one kind, but includes (1) an amorphous film, which consists only of carbon, but with the bonding form of a hybrid of sp, sp.sup.2 and sp.sup.3 ; (2) a collective mass of fine graphites; (3) an amorphous film containing hydrogen atoms other than carbon atoms (hereinafter abbreviated as a-C:H film); (4) a film containing structures of fine diamonds or fine graphites and amorphous. Also, for the respective films as mentioned above, the properties of the film will differ depending on the ratio of sp, sp.sup.2 and sp.sup.3 ; (2), the size of graphite; (3), the ratio of hydrogen to carbon; and (4) the ratio of crystal to amorphous. Particularly, a film containing much multiple bonds or a long conjugated system with nonlocalized multiple bonds or containing graphite fine crystals is liable to reduce lead oxide which is the component of glass, whereby precipitation of lead occurs, thus having the drawbacks that durability of the mold is lowered and also that plane precision of the optical element is lowered.
Accordingly, an object of the present invention is to provide a mold for an optical element molding suitable for molding of an optical element of glass, particularly a mold for optical element molding which is free from fusion with glass at high temperature, is capable of mirror polishing, has an appropriate hardness, is hardly oxidizable and free from precipitation of lead.